HEMS Low Power Carbon Nanotube Field Emission Electron Source for Chemical Ionization Mass Spectrometry

Transcription

1 Low Power Carbon Nanotube Field Emission Electron Source for Chemical Ionization Mass Spectrometry HEMS 2015 Charles B. Parker, Erich J. Radauscher, Adam D. Keil, Mitch Wells, Jason J. Amsden, Jeffrey R. Piascik, Brian R. Stoner,, Jeffrey T. Glass Department of Electrical and Computer Engineering, Duke University, Durham, North Carolina, United States FLIR Systems, Inc., West Lafayette, IN, USA Engineering and Applied Physics Division, RTI International, Research Triangle Park, NC USA Contact info: Nanomaterials and Thin Films Lab:

2 Introduction Background and Research Objectives Experimental Methods Results and Discussion Concluding Remarks

3 1 of 18 Introduction TSA Investment in Airport ETD Technology Year Amount $15 million $39 million $84 million $140 million TSA = Transportation Security Administration ETD = Explosive Trace Detection Source: House Report - Department of Homeland Security Appropriations Bills ( ) Portable instrumentation can offer rapid and accurate ETD Desire to reduce maintenance and consumables. Replace filaments with carbon nanotube field emitter. Carbon nanotube field emission electron sources demonstrate improvements in the following areas: Power requirements Cycling capability Lifetime

4 2 of 18 Carbon Nanotube Field Emission Anode Electrons Cathode (Emitter) E(x) V(x) Φ β = Local Electric Field = Applied Electric Field = Work Function = Field Enhancement Factor Spatial distribution of E-field potential near a sharp tip Advantages of CNT Emitters High electrical conductivity High chemical & mechanical stability High current densities High thermal conductivity Large aspect ratio ( >10,000 ) = High β values Large surface migration activation energy Low Thermal Expansion Coeff. Very high tensile strength

5 3 of 18 Fowler-Nordheim (F-N) Model and CNT Field Emitters A relationship between J, β, ϕ, and E can be developed using Schrodinger equation F-N model is used to characterize CNT field emitters Xu et al, J. Phys.: Condens. Matter v17 pp L507-L512 (2005)

6 Mass Spectrometry: Electron Ionization (EI) & Chemical Ionization (CI) (EI) Sample To Mass Analyzer Electron Source (CI) = Electron Beam = Sample = Reagent Gas Reagent Gas Sample To Mass Analyzer = Repeller Plate Electron Source 4 of 18

7 5 of 18 Integration and Evaluation Goals 1. Design robust & replaceable CNT electron source 2. Characterize and optimize performance 1. Compare CNT field emitters to a filament Carbon Nanotube Forest 2. Evaluate CNT source in a CI Mass Spectrometer 10 µm

8 Electron Source Platform Platform design considerations: Can be integrated into existing system Durable, can withstand voltages and pressures needed Straightforward assembly and replacement Minimal effects on field emission 6 of 18

9 7 of 18 Substrate Design and CNT Growth Doping = 200 nm phosphosilicate glass (PSG) 1 μm poly-si 500 μm n-type (100) Si (ρ = 1-2) Ω-cm) Thin conductive poly-si film used to reduce transport barrier Growth parameters tuned for improved catalyst control Lower temperature Shorter dwell times

10 8 of 18 Adhesion and Field Emission Factors Investigated adhesion techniques for FE optimization Investigated various mesh materials Bulk testing provided means of FA and correction

11 Ex-Situ Characterization Source Measure Units Device Under Test Test Bed Provides Controlled Environment Pressure Gas Species I-V Curve Analysis V to V th Lifetime Analysis V controlled I 9 of 18

12 UHV I-V Characterization Initial Improved Enhanced substrates and improved CNT growth led to the following: Superior device I-V characteristics Better control of desired emission current 10 of 18

13 Extraction Voltage (V) Extraction Voltage (V) Lifetime Characterization Initial Fixed current tests of improved CNT emitters. Operated at 10-4 Torr of lab air and a fixed 5 μa CNT emission current ~3x lifetime increase over initial CNT emitters Decreased variation in emission current and extraction fields Time (hr) Improved = failure Time (hr) 11 of 18

14 12 of 18 Performance Discussion Vto = 1.02 / V/μm Vth = 1.16 / V/μm Experimental optimization resulted in improved lifetime and superb performance I-V curves demonstrate excellent I-V performance Fowler-Nordheim plots show nonlinear characteristics, as expected

15 13 of 18 Pulsed Operation Discussion Pulse Parameters 2 sec ramp 10 sec ON 8 sec OFF Data indicates significant pulsing benefits over tungsten/rhenium filaments Cycling ON/OFF shows significantly less impact on lifetime compared to filaments Pulse Parameters 2 sec ramp 10 sec ON 8 sec OFF Assuming one pulse equals sampling of one ticket, CNTs indicate lifetime of 450,000 tickets Pulsed CNTs have 100x filament lifetime

16 CNT and Filament Source Comparison - PFTBA Used standard calibration gas (Perfluorotributylamine [PFTBA]) a Initial electron ionization (EI) performed with both sources to calibrate system Good correlation between CNT and filament spectra in EI, positive chemical ionization (PCI), and negative chemical ionization (NCI) modes b * Spectra collected at FLIR Systems, Inc. 14 of 18

17 15 of 18 CNT and Filament Source Comparison Trace Explosives Trace explosive spectra collected with CNT field emission source is nearly identical to spectra taken with the filament thermionic source Three common explosives were analyzed, calibration curves were generally linear as expected

18 16 of 18 Methods Results/Discussion Concluding Remarks Summary of Results and Discussion Designed & integrated a CNT field emission source in a chemical ionization mass spectrometry system Optimized CNT emitters to achieve low threshold voltages, long lifetime Showed benefits over standard thermionic filaments Number of Tickets (Cycles) Power Lifetime Good agreement between spectra generated with filaments and CNT field emission source for electron ionization and chemical ionization spectra.

19 17 of 18 Methods Results/Discussion Concluding Remarks Closing Information Future Studies Further improving electrical and mechanical characteristics with integration of thin film diffusion/adhesion layers Publication Radauscher, E., A. Keil, M. Wells, J. Amsden, J. Piascik, C. Parker, B. Stoner, and J. Glass, Chemical Ionization Mass Spectrometry Using Carbon Nanotube Field Emission Electron Sources. Journal of The American Society for Mass Spectrometry, 2015 dx.doi.org/ /s This work was sponsored in part by a contract with the Department of Homeland Security Science and Technology Directorate